Dylan Thurston <dpt@math.harvard.edu> points out my mistake:
> > Strictness alas matters. Here's the witness:
> >
> > class Num a => ZeroList a where
> > consZero :: a -> [a]
> > consZero _ = 0:xs
>> Err, "Num a" is already a bad context by Simon's criterion because of
> "fromInteger", which is what ultimately causes the problem in this
> case.
Quite right. I overlooked the fact that the superclass constraint
"Num a" implicitly includes all the methods of Num in ZeroList for
these purposes.
Olaf Chitil <olaf@cs.york.ac.uk> gives an example along the lines I
was attempting (which I've corrected slightly):
> instance Show MyType where
> shows _ = ("element of my type" ++ )
The important thing here is that the non-strict shows method can make
up any result it likes.
I do think some sort of extended defaulting mechanism as suggested by
Thomas Hallgren would be useful. Yes, we will occasionally choose
confusing interpretations of e.g. "" vs []. On the other hand, I'm
sure we can warn when defaulting in this way.
But its not the magic bullet against the monomorphism restriction one
might have hoped for...
-Jan-Willem Maessen